One of the most important and fundamental processes in biology is the division of cells during the cell cycle. This process ensures the controlled production of subsequent generations of cells with defined biological function. It is a highly regulated phenomenon and responds to a diverse set of cellular signals both within the cell and from external sources. Cyclin-dependent protein kinases (“CDKs”), constitute a family of well-conserved enzymes that play multiple roles within the cell, such as cell cycle regulation and transcriptional control (Science 1996, Vol. 274:1643-1677; Ann. Rev. Cell Dev. Biol., 1997, 13:261-291).
Some members of the family, such as CDK1, 2, 3, 4, and 6 regulate the transition between different phases of the cell cycle, such as the progression from a quiescent stage in G1 (the gap between mitosis and the onset of DNA replication for a new round of cell division) to S (the period of active DNA synthesis), or the progression from G2 to M phase, in which active mitosis and cell division occur. Other members of this family of proteins, including CDK7, 8, and 9 regulate key points in the transcription cycle, whereas CDK5 plays a role in neuronal and secretory cell function.
CDK complexes are formed through association of a regulatory cyclin subunit (e.g., cyclin A, B1, B2, D1, D2, D3, and E) and a catalytic kinase subunit (e.g., cdc2 (CDK1), CDK2, CDK4, CDK5, and CDK6). To date, thirteen kinase subunits have been identified in humans (Chen et al., Biochem. Biophys. Res. Commun., 2007, 354:735-40; S. Mani et al., Exp. Opin. Invest. Drugs, 2000, 9(8):1849-1870; J. C. Sergere et al., Biochem. Biophys. Res. Commun., 2000, 276:271-277; D. Hu et al, J. Biochem. Chem., 2003, 278(10):8623-8629). As the name implies, the CDKs display an absolute dependence on the cyclin subunit in order to phosphorylate their target substrates, and different kinase/cyclin pairs function to regulate progression through specific portions of the cell cycle.
CDK9 in association with its cyclin partners (cyclin T1, T2a, T2b, or K) constitutes the catalytic component of the positive P-TEFb protein kinase complex that functions during the elongation phase of transcription by phosphorylating the carboxyl-terminal domain (CTD) of the largest subunit of RNA polymerase II. P-TEFb acts in concert with positive transcription factor NfκB as well as negative transcription factors, thus overcoming a block of transcriptional elongation (Liu and Herrmann 2005).
It is known that cell-cycle dysregulation, which is one of the cardinal characteristics of neoplastic cells, is closely associated with genetic alteration and deregulation of CDKs and their regulators, suggesting that inhibitors of CDKs may be useful as therapeutics for proliferative diseases, such as cancer. Thus, small molecule inhibitors targeting CDKs have been the focus of extensive interest in cancer therapy (Current Opinion in Pharmacology, 2003, 3:362-370). The ability of inhibiting cell cycle progression suggests a general role for small molecule inhibitors of CDKs as therapeutics for proliferative diseases, such as cancer, neuro-fibromatosis, psoriasis, fungal infections, endotoxic shock, transplantation rejection, vascular smooth cell proliferation associated with artherosclerosis, pulmonary fibrosis, arthritis, glomerulonephritis and post-surgical stenosis and restenosis (U.S. Pat. No. 6,114,365). While inhibition of cell cycle-related CDKs is clearly relevant in oncology applications, this may not be the case for the inhibition of RNA polymerase-regulating CDKs. Recently, inhibition of CDK9/cyclin T function was linked to prevention of HIV replication and the discovery of new CDK biology thus continues to open up new therapeutic indications for CDK inhibitors (Sausville, E. A. Trends Molec. Med. 2002, 8, S32-S37), such as, for example, viral infections (WO 02/100401). CDK inhibitors could conceivably also be used to treat other conditions such as immunological diseases and neurodegenerative diseases, amongst others.
More than 50 pharmacological CDK inhibitors have been described, some of which have potent antitumor activity (Current Opinion in Pharmacology, 2003(3): 362-370). A comprehensive review about the known CDK inhibitors may be found in Angew. Chem. Int. Ed. Engl. 2003, 42(19):2122-2138.
The use of 2-anilino-4-phenylpyrimidine derivatives as cyclin-dependent kinase inhibitors for the treatment of e.g. cancer has been reported in WO 2005/012262. Furthermore, 2-pyridinylamino-4-thiazolyl-pyrimidine derivatives for the treatment of cancer etc. have been described in WO 2005/012298. The use of 4,5-dihydro-thiazolo, oxazolo and imidazolo[4,5-h]quinazolin-8-ylamines as protein kinase inhibitors is known from WO 2005/005438. Furthermore, indolinone derivatives and induribin derivatives, which are useful as cyclin-dependent kinase inhibitors have been disclosed in WO 02/081445 and WO 02/074742. Additionally, CDK inhibitors for various therapeutic applications have been described in WO2005/026129.
Known CDK inhibitors may be classified according to their ability to inhibit CDKs in general or according to their selectivity for a specific CDK. Flavopiridol, for example, acts as a “pan” CDK antagonist and is not particularly selective for a specific CDK (Current Opinion in Pharmacology, 2003(3): 362-370). Purine-based CDK inhibitors, such as olomoucine, roscovitine, purvanolols and CGP74514A are known to exhibit a greater selectivity for CDKs 1, 2 and 5, but show no inhibitory activity against CDKs 4 and 6 (Current Opinion in Pharmacology, 2003(3): 362-370). Furthermore, it has been demonstrated that purine-based CDK inhibitors such as roscovitine can exert anti-apoptotic effects in the nervous system (Pharmacol Ther 2002, 93:135-143) or prevent neuronal death in neurodegenerative diseases, such as Alzheimers's disease (Biochem Biophys Res Commun 2002 (297):1154-1158; Trends Pharmacol Sci 2002 (23):417-425).
Given the tremendous potential of targeting CDKs for the therapy of conditions such as proliferative, immunological, infectious, cardiovascular and neurodegenerative diseases, the development of small molecules as selective inhibitors of particular CDKs constitutes a desirable goal.
The present invention provides novel small molecule inhibitors of cyclin-dependent kinases. Preferably, said small molecule inhibitors show an increased potency to inhibit a particular CDK, in particular CDK9. Said small molecule inhibitors may have a therapeutic utility for the treatment of conditions such as proliferative, immunological, neurodegenerative, infectious and cardiovascular diseases. Furthermore, the small molecule inhibitors of the present invention have surprisingly been shown to exert a beneficial effect in the treatment of inflammatory diseases and of any type of pain.
Current treatments for pain are only partially effective, and many also cause debilitating or dangerous side effects. For example, many of the traditional analgesics used to treat severe pain induce debilitating side effects such as nausea, dizziness, constipation, respiratory depression, and cognitive dysfunction (Brower, 2000).
Although there is already a broad panel of approved pain medications like non-narcotic analgesics, opioid analgesics, calcium channel blockers, muscle relaxants, and systemic corticosteroids available, said treatments remain merely empirical and, while they may relieve the symptoms of pain, they do not lead to complete relief in most cases. This is also due to fact that the mechanisms underlying the development of the different types of pain are still only poorly understood. Researchers are only just beginning to appreciate the complexity and diversity of the signaling systems used to relay nerve impulses for each type of pain.
Generally, pain is defined as an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage, according to the International Association for the Study of Pain (IASP). Specifically, pain may occur as acute or chronic pain.
Acute pain occurs for brief periods of time, typically less than 1 month and is associated with temporary disorders. It is a natural body response to let the host be aware of physiological or biochemical alteration that could result in further damage within a short period of time. It is felt when noxious stimuli activate high threshold mechanical and/or thermal nociceptors in peripheral nerve endings and the evoked action potentials in thinly myelinated (Aδ) and/or unmyelinated (C) afferent fibres reach a conscious brain. Said noxious stimuli may be provided by injury, surgery, illness, trauma or painful medical procedures. Acute pain usually disappears when the underlying cause has been treated or has healed. Unrelieved acute pain, however, may lead to chronic pain problems that may result in long hospital stays, rehospitalizations, visits to outpatient clinics and emergency departments, and increased health care costs.
In contrast to acute pain, chronic pain persists long after the initial injury has healed and often spreads to other parts of the body, with diverse pathological and psychiatric consequences. Chronic somatic pain results from inflammatory responses to trauma in peripheral tissues (e.g., nerve entrapment, surgical procedures, cancer, or arthritis), which leads to oversensitization of nociceptors and intense searing pain responses to normally non-noxious stimuli (hyperalgesia). Chronic pain is continuous and recurrent and its intensity will vary from mild to severe disabling pain that may significantly reduce quality of life.
Chronic pain is currently treated with conventional analgesics such as NSAIDs (Ibuprofen, Naproxen), Cox-2 inhibitors (Celecoxib, Valdecoxib, Rofecoxib) and opiates (codeine, morphin, thebain, papaverin, noscapin). For a significant number of patients however, these drugs provide insufficient pain relief.
Another subtype of pain, inflammatory pain, can occur as acute as well as chronic pain. Resulting injuries of tissue and neurons must not but may develop into long-lasting chronic neuropathic pain effects in succession to such inflammatory events.
Inflammatory pain is mediated by noxious stimuli like e.g. inflammatory mediators (e.g. cytokines, such as TNF α, prostaglandins, substance P, bradykinin, purines, histamine, and serotonine), which are released following tissue injury, disease, or inflammation and other noxious stimuli (e.g. thermal, mechanical, or chemical stimuli). In addition, cytokines and growth factors can influence neuronal phenotype and function (Besson 1999). These mediators are detected by nociceptors (sensory receptors) that are distributed throughout the periphery of the tissue. Said nociceptors are sensitive to noxious stimuli (e.g. mechanical, thermal, or chemical), which would damage tissue if prolonged (Koltzenburg 2000). A special class of so called C-nociceptors represent a class of “silent” nociceptors that do not respond to any level of mechanical or thermal stimuli but are activated in presence of inflammation only.
Current approaches for the treatment of especially inflammatory pain aim at cytokine inhibition (e.g. IL1β) and suppression of pro-inflammatory TNFα. Current approved anticytokine/anti-TNFalpha treatments are based on chimeric antibodies such as Infliximab and Etanercept which reduce TNFα circulation in the bloodstream. TNFα is one of the most important inflammatory mediators that induces synthesis of important enzymes such as COX-2, MMP, iNOS, cPLa2 and others. The main drawbacks of these “biologicals”, however, reside in their immunogenic potential with attendant loss of efficacy and their kinetics that lead to a more or less digital all-or-nothing reduction of circulating TNFα. The latter can result in severe immune suppressive side effects.
A distinct form of chronic pain, neuropathic (or neurogenic) pain, arises as a result of peripheral or central nerve dysfunction and includes a variety of conditions that differ in etiology as well as location. Generally, the causes of neuropathic pain are diverse, but share the common symptom of damage to the peripheral nerves or components of central pathways. The causative factors might be metabolic, viral or mechanical nerve lesion. Neuropathic pain is believed to be sustained by aberrant somatosensory processes in the peripheral nervous system, the CNS, or both. Neuropathic pain is not directly linked to stimulation of nociceptors, but instead, is thought to arise e.g. from oversensitization of glutamate receptors on postsynaptic neurons in the gray matter (dorsal horn) of the spinal cord.
Neuropathic pain is associated with conditions such as nerve degeneration in diabetes and postherpetic neuralgia (shingles). Neuropathic pain conditions are the consequence of a number of diseases and conditions, including diabetes, AIDS, multiple sclerosis, stump and phantom pain after amputation, cancer-related neuropathy, post-herpetic neuralgia, traumatic nerve injury, ischemic neuropathy, nerve compression, stroke, spinal cord injury.
Management of neuropathic pain remains a major clinical challenge, partly due to an inadequate understanding of the mechanisms involved in the development and maintenance of neuropathic pain. Many existing analgesics are ineffective in treating neuropathic pain and most of current narcotic and non-narcotic drugs do not control the pain. Current clinical practice includes the use of a number of drug classes for the management of neuropathic pain, for example anticonvulsants, tricyclic antidepressants, and systemic local anaesthetics. However, the usual outcome of such treatment is partial or unsatisfactory pain relief, and in some cases the adverse effects of these drugs outweigh their clinical usefulness. Classic analgesics are widely believed to be poorly effective or ineffective in the treatment of neuropathic pain. Few clinical studies on the use of non steroidal anti-inflammatory drugs (NSAIDs) or opiates in the treatment of neuropathic pain have been conducted, but in those which have, the results appear to indicate that NSAIDs are poorly effective or ineffective and opiates only work at high doses. A review analysing the controlled clinical data for peripheral neuropathic pain (PNP) (Pain, November, 1997 73(2), 123-39) reported that NSAIDs were probably ineffective as analgesics for PNP and that there was no long-term data supporting the analgesic effectiveness of any drug.
Available analgesic drugs often produce insufficient pain relief. Although tricyclic antidepressants and some antiepileptic drugs, for example gabapentin, lamotrigine and carbamazepine, are efficient in some patients, there remains a large unmet need for efficient drugs for the treatment of these conditions.
In conclusion, there is a high unmet need for safe and effective methods of pain treatment, in particular of chronic inflammatory and neuropathic pain.